1. Field of the Invention
This invention relates generally to the field of coating thickness measurement, and more specifically, to a method for measuring a coating thickness using ultrasonic signals and for processing thereof.
2. Background of the Invention
There is interest in the art in improving the methods currently used for measuring the thickness of coating layers under a variety of conditions, including those wherein there is limited access to the layer itself. Such thickness measurements can be used to provide control and monitoring of the thickness of various types of coating layers such as protective layers or deposits.
Of particular importance is the thickness measurement of protective layers used in coating a bottom surface of a tank against corrosion. Rupture of a tank can have devastating consequences such as a highly negative environmental impact. Contactless and nondestructive measuring using ultrasonics permits coated regions, which are inaccessible from the outside (e.g., such regions as tank bottoms) are to be examined.
Conventional methods for measuring the thickness of a coating layer formed on a substrate include the use of ultrasonic waves. According to these methods, ultrasonic waves are generally applied to the coating layer and a substrate through a liquid medium providing ultrasonic propagation. Reflected ultrasonic waves are then detected by a suitable detector.
Coating thickness is typically measured using a technique known in the art as the ultrasonic pulse-echo method. Using this method, a coating thickness is measured based on an evaluation of the ultrasonic wave propagation delay between echoes from the lower surface of the coating layer and the upper surface of the coating layer. However, if the coating layer thickness is such that the echoes are superposed on each other or otherwise interfere with one another to produce a single signal whose characteristics cannot be resolved, this method is ineffective. Therefore, this ultrasonic pulse-echo method is, in general, only applicable to measuring only relatively thick coating layers.
One possible way to enhance measurement of thin coating layers using ultrasonic pulse-echo techniques is to use higher ultrasonic frequencies. The use of such higher frequencies assists in the evaluation of thinner layers because higher frequency signals are less difficult to resolve because of shorter wavelengths. However, at higher frequencies, surface roughness (which can produce ultrasonic scattering) and sound absorption within the layer (i.e., attenuation of the ultrasonic signal) reduce the effectiveness of this approach.
Our U.S. Pat. No. 5,942,687 to Simmonds et al., disclosed an ultrasonic measuring method for determining the thickness of a substrate in the form of a single metal layer wherein a broad band ultrasonic pulse is directed at the metal layer from an ultrasonic transducer. A Fourier analysis is performed on a return signal from the metal layer to generate a frequency domain signal, and the thickness of the metal layer is determined from the frequency domain signal. A suitable signal processing delay of at least two microseconds is provided to eliminate the initial unwanted portion of the signal.
A different technique is required for measuring the thickness of a coating material on a substrate metal layer, i.e., a technique other than the technique used for the measurement of a single metal layer disclosed in the Simmonds et al. patent. In order to determine the thickness of such a coating layer using the method of the patent, the coating material of the layer would have to be effectively separated from the metal substrate. Further, if the method uses a microsecond delay, or longer, in obtaining a return signal from a single metal layer, the method is xe2x80x9cblindxe2x80x9d to the presence of the coating layer. In addition, only a fraction of the total energy sent to the coated surface will be xe2x80x9ctrappedxe2x80x9d in the coating and reflected back to the transducer from the coating/metal interface, and the xe2x80x9ctrappedxe2x80x9d energy will pass into the metal layer.
Further differences in the physical characteristics between a metal substrate and a coating layer prevent the use of the method of the patent in measuring the thickness of a coating layer. A single layer of steel or similar metal provides a relatively low attenuation of the input signal, i.e., the corresponding signal dies slowly. However, a coating layer, which is often a polymer, provides a high attenuation of the input signal, i.e., the corresponding signal dies quickly.
The present invention is directed to a method for measuring the thickness of a coating layer on a metal substrate. A broad band of frequencies is transmitted by a transducer towards a coating layer, which is above a substrate layer and is below a fluid layer. A backscattered signal is reflected from a fluid/coating layer interface, and a trailing signal is reflected from a coating layer/substrate interface. The trailing signal reaches the transducer after a time delay relative to the backscattered signal. The trailing signal is discriminated from the backscattered signal based on the time delay. As a result, only the trailing signal will be processed. The trailing signal is deconvolved into a set of frequencies. The amplitude of each frequency of the trailing signal is then measured. The frequency, which has the greatest amplitude, is determined to be the resonant frequency of the coating layer. The resonant frequency is then used to calculate the thickness of the coating layer.
In another aspect of the present invention, an apparatus is provided for measuring the thickness of a coating layer having a resonant frequency and being deposed on a substrate between the substrate and a fluid so as to create a fluid/coating interface and a coating/substrate interface. The apparatus includes a transducer, a signal receiving device, and a signal processor. The transducer is for transmitting an incident signal comprising a broad band of frequencies towards the coating layer. The signal-receiving device receives a backscattered signal from the fluid/coating interface and a trailing signal from the coating/substrate interface after a time delay relative to the backscattered signal. The signal processor is operably associated with the signal-receiving device and establishes a signal-processing window based on the time delay such that only the trailing signal is processed. Further, the signal processor is for (i) measuring the amplitude of each frequency component of the trailing signal, (ii) determining the resonant frequency of the coating layer as the frequency component with the greatest amplitude, and for (iii) calculating the thickness of the coating layer using the resonant frequency so determined.
Other features and advantages of the invention will be set forth in, or will be apparent from, the detailed description of the preferred embodiments, which follows.